The present application claims priority to Japanese Application No. P2000-241984 filed Aug. 10, 2000, which application is incorporated herein by reference to the extent permitted by law.
The present invention relates to a thin-film semiconductor device formed by integrating thin-film transistors. More particularly, the present invention relates to a thin-film semiconductor device which has a pixel array section and a peripheral circuit section arranged thereabout and which is used as a driving substrate for a liquid-crystal display unit of active matrix type.
A liquid-crystal display unit of active matrix type usually has thin-film transistors as switching elements. In thin-film transistors, the semiconductor thin film that functions as the active layer is conventionally made of polycrystalline silicon. Thin-film transistors of polycrystalline silicon are used not only for switching elements but also for circuit elements. Combinations of switching elements and circuit elements form peripheral driving circuits on one substrate. In addition, thin-film transistors of polycrystalline silicon can be made extremely small. Therefore, each switching element occupies only a small area in each pixel, and this leads to a large aperture ratio of pixel. Liquid crystal display units of active matrix type are small in size and capable of high definition display, and they are suitable for monitors of video camcorders and digital cameras and portable telephone terminals.
Unfortunately, conventional thin-film transistors of polycrystalline silicon need a comparatively high driving voltage of about 9-15 V. This leads to an increased power consumption of liquid crystal display which is not desirable for portable telephones and any other portable devices from the standpoint of power saving. If a liquid crystal display of active matrix type is to work with a low power consumption, it is necessary that thin-film transistors of polycrystalline silicon work at a low driving voltage. This requires thin-film transistors of polycrystalline silicon, particularly those for peripheral driving circuits, to work with a large current at a low voltage.
On the other hand, thin-film transistors of polycrystalline silicon suffer another disadvantage when they are formed on a glass substrate having a glass transition point of about 600-700xc2x0 C., by so-called low-temperature process. The disadvantage is that the breakdown voltage is low due to hot carriers, particularly in the case of n-channel type transistors. One way to prevent deterioration by hot carriers is to form an LDD region (lightly doped drain region) at the drain end (at least) of thin-film transistors. The LDD region alleviates the concentration of electric field at the drain end. However, the LDD region formed at the drain end inevitably lowers the on-state current of the thin-film transistor. This is a hindrance to realizing thin-film transistors that work with a large current at a low driving voltage, thereby saving power consumption.
There has been disclosed in Japanese Patent Laid-open No. 45930/1997 a new thin-film transistor which is so constructed as to meet both requirements for preventing deterioration by hot carriers and for increasing on-stage current. The disclosed structure is characterized in that part of the LDD region overlaps with part of the electrode. However, nothing is disclosed about how the relative position of the LDD region and the gate electrode affects the operating characteristics of the thin-film transistor.
A liquid crystal display of active matrix type in which the pixel array section and the peripheral circuit section are formed on one substrate requires that the thin-film transistors for the peripheral driving circuit usually have a high on-state current. It also requires that the thin-film transistors formed in the pixel array section for switching pixel electrodes have a low leakage current (off-state current). It is natural that the thin-film transistor for circuits and the thin-film transistor for pixels differ from each other in characteristic properties required. Technologies to cope with this situation have been disclosed in Japanese Patent Laid-open Nos. 88972/1994 and 189998/1998 and WO98/13911. However, they are not satisfactory from the practical point of view.
In view of the foregoing, the present invention was completed by improving the structure of integrated thin-film transistors used for thin-film semiconductor device such as liquid crystal display of active matrix type. It is an object of the present invention to provide a thin-film semiconductor device suitable for high-quality image display with low power consumption. The thin-film semiconductor device consists of thin-film transistors for circuits that work with a low driving voltage and thin-film transistors for pixels that work with a small leakage current, both types of thin-film transistors being integrated on one substrate. The present invention is directed to a thin-film semiconductor device which has a pixel array section and a peripheral circuit section arranged around it, said pixel array section containing pixel electrodes and thin-film transistors for pixels which switch the pixel electrodes, said peripheral circuit section containing driving circuits each consisting of thin-film transistors for circuits which drive the thin-film transistors for pixels, said each thin-film transistor having the laminate structure consisting of a semiconductor thin film, a gate electrode, and a gate insulating film interposed between them, and said semiconductor thin film having a channel region -inside the end of the gate electrode, a lightly doped region outside said channel region, a heavily doped region outside said lightly doped region, and a concentration boundary which separates said lightly doped region and heavily doped region from each other, wherein said concentration boundary measured from the end of said gate electrode is positioned more inside in said thin-film transistor for circuits than in said thin-film transistor for pixels.
According to a preferred embodiment, the concentration boundary of the thin-film transistor for circuits is positioned inside the end of the gate electrode and the concentration boundary of the thin-film transistor for pixels is positioned outside the end of the gate electrode; or the concentration boundary of the thin-film transistor for circuits is positioned outside as much as a first distance from the end of the gate electrode and the concentration boundary of the thin-film transistor for pixels is positioned outside as much as a second distance from the end of the gate electrode, with the first distance being shorter than the second distance.
According to another preferred embodiment, the thin-film transistor for circuits has a lightly doped region at the drain side but does not have a lightly doped region at the source side. More preferably, each thin-film transistor has the laminate structure of bottom gate type in which the semiconductor thin film is placed on the gate electrode with a gate insulating film interposed between them.
The present invention has the following features. The concentration boundary measured from the end of the gate electrode is positioned more inside in the thin-film transistor for circuits than in the thin-film transistor for pixels. The concentration boundary separating the lightly doped region (LDD region) and the heavily doped region (drain region and source region) from each other increases on-state current as it moves toward inside from the end of the gate electrode and decreases leakage current as it moves toward outside from the end of the gate electrode. This was found by the present inventors. Adjusting the position of the concentration boundary relative to the end of the gate electrode optimizes the operating characteristics of the thin-film transistor for circuits and the thin-film transistor for pixels. The result is a reduction of power consumption in the peripheral circuit section without the pixel array section losing image quality.